MEMS technology has been under steady development for some time, and as a result various MEMS devices have been considered and demonstrated for several applications. MEMS technology is an attractive approach for providing many products, such as inertial sensors, accelerometers for measuring linear acceleration, gyroscopes for measuring angular velocity, optical devices, pressure sensors, etc. A MEMS inertial sensor typically includes a proof mass which is flexibly attached to the rest of the device. Relative motion between the proof mass and the rest of the device is driven by actuators and/or sensed by sensors in various ways, depending on the detailed device design. Other MEMS applications include optical applications such as movable mirrors, and RF applications such as RF switches and resonators.
MEMS devices typically have mechanical features that photo lithographically are defined on the surface or within the silicon substrates. There are two classes of MEMS technology, bulk silicon and surface-micromachining. In case of bulk silicon, features are created by selectively removing silicon to form the desired structures. In case of surface-micromachining, it is an additive process using polysilicon layers on top of sacrificial oxides and then by removing the sacrificial layers MEMS structures are created.
Both MEMS technologies require post MEMS processing and packaging. Most MEMS sensors and/or actuators require electrical inputs and output to perform their design functions. Traditionally MEMS devices required custom cavity based packages to provide for such input and output access. MEMS features are generally very fragile and sensitive to dusts and particles. Most MEMS features are measured in microns and require special assembly and packaging care. Such non-standard packaging has been primary contributor to the inherent high cost of many MEMS devices.
There has been a constant and growing trend in the MEMS industry for doing what is known as die-level-packaging (DLP). DLP is generally provides for a protective cover over the sensitive MEMS feature that will allow the part to be handled by standard assembly and packaging means. One common method has been to have pre-fabricated silicon covers to be picked and placed on top of the MEMS structure by automated means prior to dicing the MEMS wafers.
Yet another preferred method is to provide for protective covers by etching cavities in a silicon wafer and placing it on top of the MEMS wafer by various means, solder, glass frits, adhesives, to name just a few. This is preferred over the pick and place capping technique since it is performed at wafer level and not at device level.
However, one of the key challenges in performing any of the protective capping techniques has been to allow for easy access to the input/output pads. In the field of MEMS, it is often required that a release step is performed that exposes critical features. This release step is typically a type of etch that removes material, often silicon, in the region of interest.
One common method for making such access is to provide for some access openings in the cover wafer such that the features of interest are accessible after the cover attach. However, this requires added processing and the etching through the wafers. There are several limitations to this method. First, the number of openings per device are fairly limited due to mechanical limitations and wafer fragility in case of many openings. Secondly, the openings can not be continuous on any one side. Etching a hole through a wafer has its challenges and requires careful design and process development.
Wet etching is often performed with alkali hydroxide or similar chemistries that are known to etch silicon. The disadvantage of this method is that all materials present on the wafer must either be compatible (i.e., not etch) or they must be protected. The number of compatible materials is limited, particularly if the release takes a significant amount of time. Protecting incompatible materials requires an additional process step, and often creates process compatibility issues of its own.
Finally, another disadvantage of this method is the difficulty in masking this etch. For long, through-wafer etching, the number of materials that can be effectively patterned as a mask are quite limited.
Another potential option for such openings is by means of dry etching after cover attachment, know as DRIE. The DRIE process is relatively slow and it is performed one wafer at a time. Etching through 400 to 500 microns of silicon cover can take a significant amount of time and is also a fairly costly process. Dry etching has also successfully been used, particularly deep reactive ion etching, as it can achieve high aspect ratio etching, can be masked easily, and has been demonstrated to have high selectivity. However, a DRIE system is costly, requiring significant capital expenditure. Furthermore, a DRIE process is time consuming when removing significant (>200 μm) amount of silicon. Accordingly, the throughput of the machine utilized for etching is significantly limited.
Dry etching requires patterning a masking material to define the feature(s) to be released, similar to wet etching, and has the same problems associated therewith.
Yet another method of releasing a feature is by sawing of the portion of cover wafers that are not protecting the MEMS features and obscuring the access to the input/output features. The problem with this approach is that it is difficult to control the depth of the saw into the cover wafer accurately. In addition, removal of the unwanted silicon tabs after sawing is often destructive and potentially damaging. Alternatively, a clearing channel can be etched into the cover wafer to allow for sawing operation, but this is also problematic. Since the saw slurry will get under the silicon and over the contact pads which results in poor contact and wire bond assembly. Also, having large pieces of silicon tabs flying off the wafers during the saw operation is not considered standard and often results in damage to the saw blades.
Accordingly, what is needed is a system and method for releasing a MEMS cover-structure which should be cost-effective, and easily implemented and adaptable to existing assembly and packaging tools. The present invention addresses such a need.